Retroreflective materials have the ability to redirect light incident on the material back toward the originating light source. This property has led to the widespread use of retroreflective sheeting for a variety of traffic and personal safety uses. For example, retroreflective sheeting is commonly employed in a variety of articles, such as, road signs, barricades, license plates, pavement markers, and marking tape, as well as retroreflective tapes for vehicles and clothing.
Two known types of retroreflective sheeting are microsphere-based sheeting and cube corner sheeting. Microsphere-based sheeting, sometimes referred to as “beaded” sheeting, employs a multitude of microspheres typically at least partially embedded in a binder layer and having associated specular or diffuse reflecting materials (e.g., pigment particles, metal flakes, vapor coats, etc.) to retroreflect incident light. Due to the symmetrical geometry of beaded retroreflectors, microsphere based sheeting exhibits the same light return regardless of orientation (i.e., when rotated about an axis normal to the surface of the sheeting). Therefore, it is said that the distribution of light returned by beaded retroreflective sheeting is generally rotationally symmetric. Thus when viewing or measuring the coefficient of retroreflection (“RA”, typically expressed in units of candelas per lux per square meter) at presentation angles from 0 to 360 degrees, or when measuring at orientation angles from 0 to 360, there is relatively little variation in the retroreflectivity of beaded sheeting. For this reason, such microsphere-based sheeting has a relatively low sensitivity to the orientation at which the sheeting is placed on a surface. In general, however, such sheeting has a lower retroreflective efficiency than cube corner sheeting.
Cube corner retroreflective sheeting, sometimes referred to as “prismatic” sheeting, typically comprises a thin transparent layer having a substantially planar first surface and a second structured surface comprising a plurality of geometric structures, some or all of which include three reflective faces configured as a cube corner element. Cube corner retroreflective sheeting is commonly produced by first manufacturing a master mold that has a structured surface, such structured surface corresponding either to the desired cube corner element geometry in the finished sheeting or to a negative (inverted) copy thereof, depending upon whether the finished sheeting is to have cube corner pyramids or cube corner cavities (or both). The mold is then replicated using any suitable technique such as conventional nickel electroforming to produce tooling for forming cube corner retroreflective sheeting by processes such as embossing, extruding, or cast-and-curing. Known methods for manufacturing the master mold include pin-bundling techniques, direct machining techniques, and techniques that employ laminae. These microreplication processes produce a retroreflective sheeting with prismatic structures that have been precisely and faithfully replicated from a microstructured tool having a negative image of the desired prismatic structure.
Prismatic retroreflective sheeting, in contrast to beaded retroreflective sheeting, is generally rotationally non-symmetric. Thus, when viewing or measuring RA at presentation angles from 0 to 360 degrees, or when measuring at orientation angles from 0 to 360, there is significant variation in the retroreflectivity of prismatic sheeting. Therefore, prismatic sheeting has a higher sensitivity to the orientation at which the sheeting is placed on a surface than beaded sheeting.